Blind legal activist Chen Guangcheng, a key Chinese human rights advocate, has escaped from house arrest and issued a dramatic video plea to Premier Wen Jiabao to investigate his illegal detention and mistreatment.

Mr. Chen, who had been kept incommunicado by teams of security men for 19 months since his release from jail, was “100 percent safe in Beijing,” according to a US activist who said he had been in touch with Chen’s family.

A spokesman for the US Embassy in Beijing refused to comment on reports that Chen had sought refuge there.

Chen’s dramatic escape and the detailed allegations of beatings by police that he made in a video aired on YouTube Friday pose a new challenge for China’s leaders, already struggling with the fallout from the scandal surrounding Bo Xilai in the run-up to a leadership transition next autumn.

Chen, wearing his customary dark glasses and speaking directly to the camera, urged Mr. Wen to “thoroughly investigate … all this illegal conduct” by local officials in Linyi in Shandong province, whom he said had kept him prisoner in his home and repeatedly beaten him, his wife, and his mother.